High efficiency synthesis of Nd:YAG powder by a spray co-precipitation method for transparent ceramics

Accepted Manuscript Title: High efficiency synthesis of Nd:YAG powder by a spray co-precipitation method for transparent ceramics Authors: Wei Jing, Fang Li, Shengquan Yu, Xiangbo Ji, Tao Xu, Jian Zhang, Zhongben Pan, Zerui Yuan, Bin Kang, Jianguo Deng, Wenlong Yin, Hui Huang PII: DOI: Reference:

S0955-2219(17)30868-3 https://doi.org/10.1016/j.jeurceramsoc.2017.12.059 JECS 11664

To appear in:

Journal of the European Ceramic Society

Received date: Revised date: Accepted date:

3-7-2017 15-12-2017 27-12-2017

Please cite this article as: Jing W, Li F, Yu S, Ji X, Xu T, Zhang J, Pan Z, Yuan Z, Kang B, Deng J, Yin W, Huang H, High efficiency synthesis of Nd:YAG powder by a spray co-precipitation method for transparent ceramics, Journal of The European Ceramic Society (2010), https://doi.org/10.1016/j.jeurceramsoc.2017.12.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

High efficiency synthesis of Nd:YAG powder by a spray co-precipitation method for transparent ceramics Wei Jinga,b, Fang Lic, Shengquan Yua,b,*, Xiangbo Jia, Tao Xua,b, Jian Zhangd, Zhongben Pana,b, Zerui Yuana,b, Bin Kanga,b, Jianguo Denga,b, Wenlong Yina,b, Hui Huanga,* a

Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900,

China Sichuan research center of new materials, Chengdu 610200, China

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Research Center for Laser Fusion, China Academy of Engineering Physics, Mianyang 621900,

China

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

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*Corresponding author. Tel.: +86 028 6572 6195 E-mail addresses: [email protected] (S. Yu), [email protected] (H. Huang)

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b

Abstract

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A spray co-precipitation method was developed to efficiently synthesize Nd:YAG

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nano-powders. The effects of spray speeds and solution concentrations on the crystallization processes of calcined precursors have been studied. The results indicated that the pure phase of

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YAG could be obtained by three different crystallization processes owing to different homogeneity levels of Y and Al mixing. Pure YAG powder was obtained at 850°C and the phase purity persisted

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to 1600°C. Using the obtained powders, transparent ceramics with the in-line transmittance up to 80.2%@400 nm and 83.1%@1064 nm were fabricated by gel-casting method and hot isostatic pressing sintering. Furthermore, the microstructure and laser properties of the transparent ceramics

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have been measured. The maximum laser output of 7.015 W has been obtained with an oscillation threshold and a slope efficiency of 0.235 W and 59.4%, respectively.

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keywords: Nd:YAG; Transparent ceramics; Co-precipitation synthesis; Laser properties; Gel-casting

1. Introduction

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Yttrium aluminum garnet (Y3Al5O12, YAG) with a cubic structure in the space group Ia3d,

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has been proved to be on top of the list of the recommended laser materials. Compared with its single crystal counterpart, polycrystalline YAG ceramics have many advantages, such as the simple synthesis process, low cost, high concentration doping, and better homogeneity of doping ions [1-10]. One of the most essential points for the fabrication of high quality YAG transparent ceramics is to prepare the fine powder with high purity, fine particle size, narrow size distribution, good dispersion without hard agglomeration, and high sintering activity [11]. Usually there are two typical methods to prepare YAG powders for transparent ceramics. *Corresponding author. Tel.: +86 028 6572 6195 E-mail addresses: [email protected] (S. Yu), [email protected] (H. Huang)

The conventional method is first ball milling of Al2O3 and Y2O3 powder mixture, and the fine mixed submicron powders are then solid-state reactively sintered under vacuum or other atmospheres. However, this approach has its Achilles' heel, which is the unavoidable impurity introduced by ball grinding process. The impurities can induce lattice defects and scattering centers. Moreover, the grain size is difficult to control because of the long time sintering and the very high temperature. The other way is wet-chemical method, such as sol-gel process [12-18], co-precipitation [2, 19-30], spray pyrolysis [31], combustion synthesis [32, 33], and hydrothermal method [34, 35]. Among these techniques, the co-precipitation is one of the most recommended

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techniques, which has the advantages of simple process, low cost, excellent chemical homogeneity, good crystallinity, and pure phase at low temperature. By adopting this approach, the Y and Al can be mixed at atomic level without the drawbacks of the solid-state reaction method, which is

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benefit for the homogeneity of the powder and the quality of final transparent ceramics.

Although the preparation of YAG powder through the co-precipitation method has been widely investigated, it still needs to be improved because a practical, reliable, and widely accepted specific solution has not been reached yet. The main difficulty lies in the different precipitation

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velocity between Y3+ and Al3+ and the atomic level homogeneous mixing according to the

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stoichiometric ratio. To avoid these problems, the dripping speed is usually set to a very slow rate [19, 36-38]. However, this leads to a very low synthetic efficiency and is time consuming. In

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addition, the crystallization processes from the precursors to YAG powder were highly variable

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and the mechanism behind these traits has been rarely reported. In this work, the homogeneous Nd:YAG powders have been prepared by a high-efficiency and modified spay co-precipitation process from a mixed solution of aluminum and yttrium

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nitrates. The effects of cation concentrations of the solution and spray droping speeds on crystallization processes of the Nd:YAG precursors were investigated particularly. The mechanism

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behind crystallization process of the co-precipitated YAG powder was discussed. Besides, the well dispersed Nd:YAG nano-powders were obtained by optimizing the co-precipitaiton and calcination schedules. The high transparent Nd:YAG ceramics were fabricated by hot isostatic

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pressing sintering using as-prepared nano-sized powders. The fine microstructure and laser performance of the ceramics also proved the effectiveness of this new improved method. 2. Experimental procedure 2.1 Powder synthesis and characterization

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The aluminium nitrate nonahydrate Al(NO3)3.9H2O (Guaranteed reagent, Shanghai Ling

Feng Chemical Reagent Co. Ltd), Y2O3 (99.99%, Alfa-Aesar), and Nd2O3 (99.99%, Alfa-Aesar) were used as starting materials to prepare the salt solution. The Y2O3 and Nd2O3 were dissolved in diluted nitric acid. (HNO3, 69% Sigma–Aldrich, The nitric acid was diluted to 23% before using.) In order to promote the dissolution and remove the excess acid, the suspension was heated to boiling until approximately 90% (in volume) of the solution was evaporated. After cooling to room temperature, the distilled water was added to obtain the Y(NO3)3 and Nd(NO3)3 solution 2

required. The Al(NO3)3 solution was obtained by dissolving Al(NO3)3·9H2O in deionized water. The concentration of Al3+, Nd3+, and Y3+ was strictly determined by ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometer) analysis, performed on an ICP-OES Vista-Pro (Varian). Then the nitrates were mixed together in distilled water, according to the stoichiometric ratio of 1 at% Nd:YAG. Ammonium hydrogen carbonate (NH4HCO3), named AHC (Guaranteed reagent, Shanghai Ling Feng Chemical Reagent Co. Ltd) and ammonium sulfate ((NH4)2SO4, GR, Shanghai Ling Feng Chemical Reagent Co. Ltd) were used as precipitant. Concentration of the AHC solution was selected as 1.5 M and it was made by dissolving AHC and (NH4)2SO4 into the

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distilled water together. The initial pH of the AHC solution was adjusted to 8.1 (measured by FE20 FiveEasy Plus, Mettler Toledo) with an addition of the diluted nitric acid (69%,

Sigma–Aldrich, diluted to 23%) before the beginning of the precipitation process. For better

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cation homogeneity, the reverse-strike precipitation (adding salt solution to the precipitant one) method was selected. The salt solution was added to precipitant solution via a pneumatic spray

device. A typical co-precipitation device was designed for the experiment as shown in Fig. 1. The mechanism was similar to ultrasonic generation of micro-sized droplets [39]. The method was

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based on the generation of micro-sized droplets. Compared to conventional reverse strike

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technique, the single droplet was turned into many micro-sized droplets. The small micro-droplets could weaken the heterogeneous local concentration in the liquid system and uneven nucleation.

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Furthermore, it could reduce the segregation of Y3+ and Al3+ on account of their different

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precipitation reaction rates. These were beneficial to uniform cations mixing and the stoichiometry of the sediment.

Precipitation was performed on a IKA EUROSTAR stirrer, in a water bath in order to

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maintain the reaction temperature 25°C constant. Then, the mixed solutions with different cation concentrations (0.1 M, 0.2 M, 0.4 M) were sprayed into the precipitant solution under vigorous

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stirring at corresponding different jetting velocities (40 ml/min, 20 ml/min, 10 ml/min) controlled by a peristaltic pump. The resultant suspensions were aged for 1 h at room temperature without stirring, and then were filtered and washed twice using ethanol in sequence. Unlike conventional

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co-precipitation method, the water washing was not applied in this experiment in order to avoid the loss of cations.

After alcohol washing, the resulting products were dried at 60°C in vacuum for 24 h. The

obtained YAG precursors were calcined at different temperatures for 2 h in air. Thermal behavior

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of the precipitates was determined using thermogravimetry analysis and differential thermal analysis (Netzsch, STA 449C) from room temperature to 1400°C in flowing air (5 °C/min). Phase analyses using X-ray diffraction technique (XRD, Bruker AXS SA D2, Cu/Ka radiation) were performed on the precipitate before and after heat treatments. Dispersion state and morphologies of the YAG powders were examined using field emission scanning electron microscopy (FESEM, JSM-6700, JEOL, Japan). 2.2 Ceramic fabrication and characterization 3

The powders calcined under 1200°C for 2 h were sieved through a 100-mesh screen for 2 times. A water-soluble co-polymer named PIBM (iso-butylene and maleic anhydride, Isobam-104, Kurarary, Osaka, Japan) served as gelling agent. First, 1 wt% polyacrylic acid (purity >99.0%, Open Chemicals Co., LTD, Guangzhou, China) served as dispersant agent and 1.5 wt% PIBM were dissolved in deionized water. Then, the Nd:YAG powder was gradually added in. 0.2 wt% tetraethyl orthosilicate (TEOS, Sigma–Aldrich) and 0.2 wt% magnesium nitrate (MgO dissolved in diluted nitric acid) were also added as sintering agents. Afterwards, the mixture was ball-milled for 2 h using zirconia balls (Fritsch, Germany) to achieve homogenous slurry with 50 vol% solids

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loading in pre-mixed solution. After that, the slurry was degassed with a vacuum de-bubbling

machine (ARV-310, Thinky, Tokyo, Japan) for 30 min to produce bubble-free slurry. The obtained slurry was cast into molds at room temperature and gelled at 30°C in a constant temperature and

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humidity chamber for 72 h. The wet Nd:YAG green bodies were demolded and further dried at

60 °C for 48 h. Then the green bodies were debindered at 800 °C for 8 h in a muffle furnace with a heating rate of 1.0 °C/min. Ultimately, the achieved green bodies were sintered at 1700 °C for 30 h under vacuum at about 4×10-4 Pa. Afterwards the samples were hot isostatically pressed (HIP) at

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1700 °C and 200 MPa in argon atmosphere for 2 h. Finally, annealing was conducted at 1450 °C

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for 10 h in air for completely removing internal stresses and eliminating oxygen vacancies. 2.3. Laser experiment

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The schematic diagram of the laser experimental setup is shown in Fig. 2. The 1 at.%

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Nd:YAG transparent ceramic was cut into an aperture of 4 × 4 mm2 and a thickness of 4 mm. Both faces of the ceramic were mirror-polished to laser quality and anti-reflectively coated for both the pump and laser wavelengths at 808 nm and 1064 nm, respectively. Then, the laser ceramics was

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mounted in a Cu-holder and Indium foils were used to provide improved thermal contact from all 4 lateral sides. A laser diode with the emission wavelength at 808 nm was used as the pump source.

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The laser cavity consisted of two mirrors (M1 and M2). The concave mirror (M1) was a high-reflectivity mirror at 1064 nm, and M2 was an output mirror with a certain transmittance at 1064 nm. The output powers were measured using a laser power meter.

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3. Results and discussion

In this experiment, concentration of the AHC solution was selected as 1.5 M. The chemical

reactions of AHC hydrolysis are:

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NH4HCO3 ＋ H2O = NH4OH ＋ H2CO3 H2CO3 = H+ ＋ HCO3 HCO3 = H+ ＋ CO32  Owing to the high CO32− concentration in the AHC solution, Al3+ ions may mainly

precipitate as ammonium dawsonite [(NH4Al(OH)2CO3)] rather than pseudo-boehmite (AlOOH). In addition, Y3+ may most likely react with the present carbonate anions containing in AHC solution to form normal carbonate of [Y2(CO3)3·nH2O(n = 2 – 3)] or basic carbonate of [Y(OH)CO3]. 4

From the previous studies [1, 39], the component of YAG powder precursor produced by the reverse-strike method with AHC had two possibilities: a mixture of aluminum and yttrium precipitation respectively or a composite compound, which was determined by the specific experimental conditions. This is because the specific composition of the YAG precipitate is affected seriously by the experimental factors. However, one can be made with certainty that the beginning reaction between Y3+ and Al3+ with the precipitant is parallel, independent, and has different reaction velocities. Experiment parameters such as pH value, temperature, solution concentration, etc. will affect the reaction rates and this will make a segregation of Y3+ and Al3+,

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eventually lead to the ratio of Y3+ and Al3+ deviating from 3:5 in local micro areas. Thus, some secondary phases could appear in the calcined powder. Hence, we kept pH value, reaction

temperature, and other experiment parameters constant and take cation concentration and adding

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speed as variables to study the impacting mechanism and the optimal process.

3.1 Effect of cation concentration on the Morphology of the Crystallization of Precursors In this work, three types of aluminum salts and adding speeds were used. The ratios of the

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three aluminum salts used are shown in Table I.

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The thermal analysis curves for the dried S1, S2, and S3 precursor powders are shown in Fig. 3A, B and C, respectively. All the samples present similar weight loss ratio about 50%. And

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all the significant weight loss appeared in the range of 100-700°C associated to one endothermic

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peak at about 130°C for S2 and S3, two endothermic peaks at 127.6°C and 196.7°C for S1. The peaks at about 130°C may be caused by the loss of ammonia groups, the evaporation of absorbed water and the release of molecular water. An endothermic at 196.7°C was observed on

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the sample S1. This temperature range may be compatible to the dehydration of aluminium trihydrates [10], yttrium hydroxides, and yttrium carbonate which decompose at higher

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temperature.

The three samples also differ in the higher temperature regime. A sharp exothermic peak at about 920°C was observed on the sample S1; an exotherm at about 922°C was also presented by

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the sample S2 followed by a broad and slightly intense exothermal effect at higher temperature. This can be devoted to formation of the intermediate phase YAP which is also evidenced by the XRD results in Fig.4 and Fig.5. The exothermic peaks at 1047.2 and 1043.7°C for S1 and S2 respectively, lead to the formation of YAG which also agrees with the XRD results. For S3, no

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exothermic peaks are found from 700 to 950°C, however, two arched endothermic peaks at 983.6°C and 1118.7°C have been observed which can be attributed to the grain growth of YAG. In order to study the thermal decomposition of the YAG precursor during the calcination,

the precursors of S1, S2, and S3 have been calcined for 2 h in air at different temperatures. These heat-treated powders have been characterized by XRD. Precursor S1 showed a very slow kinetics of transformation to YAG (Fig. 4). The unheated precursor was partially crystallized with only one diffraction peaks at 10° and the crystallized phase was considered to be the yttrium carbonate 5

hydrate which had been reported by Ji-Guang Li [1]. The partial crystallization of the unheated precursor can be due to the concentration of Y atom and nonuniform atomic level cations mixing. On accounts of the evaporation of crystal water, the crystallized yttrium carbonate hydrate degenerate and the heat-treated powders were found to be amorphous with temperatures up to 300 ºC. At 600ºC, small peaks corresponding to bcc yttria (JCPDS card no. 041-1105) can be indexed and the peaks enhanced further when the temperature increased to 800 ºC. At 900ºC, mainly cubic yttria phase and the crystallized hexagonal yttrium aluminum perovskite (YAP, YAlO3, JCPDS card no. 016-0219) phase were detected with traces of the YAG phase (JCPDS card no.033-0040).

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At 1000ºC, the YAP phase disappeared and the powder consist of dominated YAG phase with the

minor phases of yttria and YAM (yttrium aluminum monoclinic, JCPDS card no. 010-1336). With further increasing of temperature to 1200 ºC and 1400 ºC, the only difference observed was the

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increase of the crystallinity of the powder which could be certified by the enhancement in the

relative intensity of diffraction peaks. When the temperature was finally increased to 1600ºC, all of the yttria and YAM convert to YAG and pure YAG without any secondary phase was obtained.

although the atomic level cation mixing was nonuniform.

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These phenomena indicated that the stoichiometric proportion of the precursor were fitted for YAG,

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For precursor S2, a very weak diffraction peak at 10° was also observed when the precursor was not heated (Fig. 5A). This indicated that the concentration of Y atom was still existed.

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Nevertheless, the transformation kinetics was different from S1 and the intermediate phase of

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YAM was not detected throughout the phase transformation process. The powder obtained at 900°C consisted of YAG phase and another intermediate phase YAP. When the temperature increased to 1000ºC, only diffraction peaks corresponding to YAG phase can be detected. With

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further increase of temperature to 1100 ºC, the differences observed were the increase of the crystallinity of powders and the appearance of a very small amount of yttria phase, which can be

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certified by the increase in the relative intensity of diffraction peaks. When increasing to 1200°C and 1300°C, improvement in the YAG and trace yttria crystallization were observed with peak refinement on the XRD patterns. Complete conversion required a calcination temperature of

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1400°C, and the phase purity persisted to 1600°C. Fig. 6 shows XRD patterns of the calcination products of precursor S3. For Precursor S3, no

obvious diffraction peaks were observed even when the particles were heated at 800°C. It can be concluded that the co-precipitated powder was amorphous until about 800°C. With further

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increase of temperature to 850 ºC, the XRD results (Fig. 6) revealed that crystallization of the amorphous precursor started and all peaks pointed to the YAG phase. Calcining at 900 °C resulted in a further transformation to YAG and the phase purity persisted to high temperatures. The distinction of crystallization processes can be owed to the distinction of the mixing uniformity of Y3+ and Al3+ at atomic level. And the intrinsic reason lies in the different precipitation reaction velocities between Y3+ and Al3+. Compared with traditional reverse-strike technique, a lot of micro-sized droplets generated by the spraying nozzle can limit the influence 6

from the distinction of the reaction rates on inhomogeneous mixing. In the meantime, it can significantly improve the work efficiency. The cation concentration affects reaction rate greatly during the three experiments. The lower the cation concentration is, the slower the reaction rate becomes, and the inhomogeneous mixing from distinction of reaction rates is gradually diminished. From the above XRD results, the peak at 10° corresponding to yttrium carbonate hydrate can be regarded as a signal of heterogeneous mixing. The precursor S1 has the worst uniformity, impurity phases such as YAM, Y2O3, and YAP have appeared through the crystallization process. Also, the endothermic peak at 196.7°C as shown in Fig. 3A could be more

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explicitly ascribed to yttrium carbonate which decomposed at higher temperature. By contrast,

no intermediate phase has appeared in the process of precursor S3 crystallization since the pure

YAG phase was detected at 850°C. Besides, no endothermic peaks at about 200°C were detected

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for both S2 and S3 samples in the DSC-TG testing. Taken together, the DSC-TG and XRD results revealed that different cation concentrations and spray speeds could lead to different

crystallization processes and morphologies of particles, though all the three as-synthesized precursors could be converted to pure YAG phase after being calcined at 1600 °C for 2 h. These

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differences could be attributed to the different homogeneity levels of Y and Al mixing. The

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spraying process parameters like S3 were found to reduce the segregation of Y3+ and Al3+. From Fig.7, the resultant powders calcined at 900°C are agglomerated and show similar

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overall morphology to that of the precursors. This is primarily due to that the YAG crystallization

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is just beginning to take shape at around 900°C, which is also demonstrated by the XRD results as shown in Fig. 4, 5, and 6. For the powders calcined at 1200°C and 1350°C, appreciable particle growth from 100 nm to 300 nm occurred with increasing the calcination temperature. From Fig.

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7(g), (h), and (i), we can clearly see that the powder of S1 is filled with obvious particle agglomerations. By contrast, the powder S2 is much better dispersed than S1, and the powder S3

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is in between. When the calcine temperature reached 1350°C, it can be noted that the obvious sintering neck and grain growth were observed in all samples, caused by high calcination temperature.

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3.4 Sintering behavior of the YAG powder and ceramic properties Based on the above results, it is considered that the resultant powder S3 will be favorable

for transparent ceramic fabrication. Therefore, only the YAG powders obtained from precursor S3 were used for the sintering study. A novel gel-casting method was applied to obtain homogeneous

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green body with high density as mentioned above. The transmittances of the Nd:YAG ceramic is 83.12% at lasing wavelength of 1064 nm as

demonstrated in Fig. 8(a). Moreover, the morphologies of the polished surface and the fracture surface were characterized by field-emission scanning electron microscopy as shown in Fig. 8 (b) and (c). It can be seen that high transparent Nd:YAG ceramics can be obtained using the as-prepared Nd:YAG powders, and no pores were detected in structure on the polished and the fracture surfaces. The grain boundaries are clean and there are almost no secondary phases at grain 7

boundaries or inner grains. Fig. 9 shows the laser output powers of the Nd:YAG ceramic sample at 1064 nm versus the pump powers at 808 nm. Two separate output couplers were experimentally utilized for evaluating laser performance of our sample with the transmittance of T1 = 10 % and T2 = 20 % respectively at the laser wavelength. By using the linear fitting of the output power data, the pump threshold power of the Nd:YAG ceramic disk was 235 mW. Working with the separate output couplers T2, the maximum output power 7.015 W was obtained and the corresponding slope efficiency was 59.4%. However, when using the output couplers T1, the output power and the slope efficiency

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were slightly decreased to 6.895 W and 58.2%, respectively. The efficiencies were close to the

Konoshima and FIRE samples as reported by A. A. Kaminskii and etc. [41]. Furthermore, in the

two experiments the output powers were approximately increased linearly with the incident pump

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powers improving. The decrease of the efficiency probably was attributed to the heating effects of the Nd:YAG ceramic sample. 4. Conclusion

Highly sinterable nanocrystalline YAG powders with homogenous dispersion were

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efficiently synthesized via a spray co-precipitation with AHC as precipitation agent. The results

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showed that compositions of the S3 powder were uniformly mixed and have a high non-crystallizing degree. Pure YAG phase could be obtained at a comparatively low temperature

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of 850 °C and there was no intermediate phase detected throughout the calcining process.

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The particle morphologies calcined at various temperatures were revealed by FESEM. The S3 powder calcined at 1200°C with a primary particle size of about 150 nm has already been successfully used to fabricate high quality Nd:YAG ceramics. The in-line transmittance reached

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80.2%@400nm and 83.1%@1064 nm respectively. The maxium laser output of 7.015 W has been obtained with an oscillation threshold and a slope efficiency of 0.235 W and 59.4%, respectively.

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The results indicated that the spray co-precipitation method was beneficial to improving the synthetic efficiency and particle properties. Acknowledgments

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This research is supported by the Key Laboratory of Science and Technology on High

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Energy Laser, CAEP.

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synthesis route to yttrium aluminum garnet (YAG) transparent ceramics, Journal of the European Ceramic Society 32(11) (2012) 2971-2979. [25] J. Su, Q.L. Zhang, S.F. Shao, W.P. Liu, S.M. Wan, S.T. Yin, Phase transition, structure and luminescence of Eu:YAG nanophosphors by co-precipitation method, Journal of Alloys and

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Compounds 470(1-2) (2009) 306-310. [26] S. Tong, T. Lu, W. Guo, Synthesis of YAG powder by alcohol–water co-precipitation method, Materials Letters 61(21) (2007) 4287-4289. [27] Z.-H. Chen, Y. Yang, Z.-G. Hu, J.-T. Li, S.-L. He, Synthesis of highly sinterable YAG nanopowders by a modified co-precipitation method, Journal of Alloys and Compounds 433(1–2) (2007) 328-331. [28] G.G. Xu, X.D. Zhang, W. He, H. Liu, H. Li, R.I. Boughton, Preparation of highly dispersed 10

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Fig. 1. schematic drawing of the designed typical co-precipitation device.

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Fig. 2. The schematic diagram of the laser experimental setup

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Fig. 3. TG–DSC curves of precursor powders. (A: sample S1, B: sample S2, C:sample S3)

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Fig. 4. XRD patterns of the S1 precursor and the samples calcined for 2 h in air at different

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temperatures: precursor, 300 ºC, 600 ºC, 800 ºC,900 ºC,1000 ºC,1200℃, 1400ºC and 1600 ºC.

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Fig. 5. XRD patterns of the S2 precursor and the samples calcined for 2 h in air at different

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temperatures: precursor, 300 ºC, 800 ºC,900 ºC,1000 ºC,1200℃, 1300ºC,1400ºC and 1600 ºC.

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Fig. 6. XRD patterns of the S3 precursor and the samples calcined for 2 h in air at different temperatures: precursor, 300 ºC, 700 ºC, 800 ºC, 850 ºC,900 ºC,1000 ºC,1200℃, 1350ºC and 1600

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ºC.

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Fig. 7. FESEM micrographs of as-synthesized precursor and the resultant powders obtained by calcining the precursors at different temperatures for 2 h: (a) precursor S1;(b) precursor S2; (c) precursor S3; (d)S1 900°C; (e) S2 900°C; (f) S3 900°C; (g) S1 1200°C; (h) S2 1200°C; (i) S3

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1200°C; (j) S1 1350°C; (k) S2 1350°C; and (l) S3 1350°C

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(a)

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Fig. 8. In-line transmittances of Nd:YAG bodies sintered at 1700°C using obtained nano-powder

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S3(a) and SEM images of surface (b) and cross section (c) for obtained Nd:YAG ceramics.

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Fig. 9. C.W. laser output power as a function of absorbed pump power.

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Table 1 The cation concentrations and adding speeds of the three different samples Cation concentration (mol/L)

Adding speed(ml/min)

S1

0.4

10

S2

0.2

20

S3

0.1

40

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Sample

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